The present invention relates to a diaphragm carburetor, more particularly for two-stroke motors, with a mixing passage connected at the inlet side to an air filter, the air being sucked in from the inlet side and the fuel being sucked in from a fuel chamber, said fuel chamber being partitioned off by a control diaphragm
A number of compensating arrangements for diaphragm carburetors for two-stroke motors in particular, as well as chain saws, clearance saws, cutting-off machines etc. have been produced. The diaphragm partitions a space, often called the measuring chamber, in the carburettor into a chamber containing fuel and a chamber containing air, hereinafter called the compensating chamber. The fuel chamber is connected on one hand to the fuel supply and on the other hand to the mixing passage of the carburettor via distributors.
The compensation is primarily intended for altering the mixing conditions when the air filter is being polluted. The increasing pollution of the filter causes an increased vacuum on the inlet side of the mixing passage. Further, the amount of combustion air decreases with an increase of the degree of pollution, leading to a richer fuel/air mix, i.e. the amount of fuel increases with respect to the amount of air. A richer mix leads to incomplete combustion and a loss of power of the engine.
In order to compensate for this, the diaphragm in the carburettor is controlled by the increased vacuum in the space between the filter and carburettor through a passage between them. In some cases the passage comprises an external hose with consequent problems of leakage at the connections, breakage of the hose etc. Other solutions for the passage have involved including it in the carburettor casing through small drilled holes in the material. Some passages are closed with pieces of pipe impressed in the carburettor casing in order to obtain the inlet to the passage in the middle of the mixing passage. The pipe inlet is often directed against the direction of flow in the mixing passage in order to obtain as great a pressure difference as possible between both sides of the control diaphragm, fuel chamber and compensating chamber.
Tests of compensating passages have shown that in certain cases it is necessary to introduce chokes into the passages in order to level out the pressure variations which occur in the suction system. Here there is also the problem of arranging these chokes, which is often done by pressing small throttling bushing into the compensating passages. This too is relatively complicated.
As it is very difficult to determine theoretically how the compensating passages should be designed for each type of motor and filter arrangement, one is largely forced to try them out. This means that the design of the passage in or on the carburetor, and the size and location etc. of the chokes may need to be changed.
Another aspect is that one type of carburetor is used for motors with different passage design requirements. The adjustment of the carburettor takes place by setting and/or changing adjusting screws in accordance with the mixing conditions. However it is not certain that the same design of compensating passage and possible chokes is suitable for the different motor and filter arrangements. This means that factory-made carburettors have to be configured for a specific motor type/area of application which in turn results in increased production and spare parts costs.